You did not give CashbackDiscount.co.uk permission to your email address in Facebook
Without your permission to your email address, it is not possible to log in on CashbackDiscount.co.uk with your Facebook account. Please remove CashbackDiscount.co.uk in your Facebook account settings and log in again.
In Facebook go to: Settings & Privacy - Settings - Apps and websites and click on the button Remove behind CashbackDiscount.co.uk.
After these steps, please try again to login in with Facebook on CashbackDiscount.co.uk and make sure to give CashbackDiscount.co.uk permission to your email address.